1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor and manufacturing method thereof.
2. Related Arts
A semiconductor acceleration sensor having the differential-capacitance type semiconductor acceleration sensor in PCT WO 92/03740 which utilizes polysilicon as an electrode is disclosed. This kind of sensor is described with reference to FIGS. 18 and 19. FIG. 18 shows a plan view of the sensor and in FIG. 19 a cross sectional view taken along line E--E of FIG. 18 is shown.
A beam structure 63 is disposed on a silicon substrate 41. The beam structure 63 made of polysilicon is constructed of anchor sections 43, 44, 45, and 46 and movable section 42, disposed and spaced at a prescribed distance above silicon substrate 41. Further, movable section 42 comprises beam sections 47 and 48, a weight section 49, and a movable electrode section 50. That is, beam sections 47 and 48 are extended from anchor sections 43, 44, 45, and 46, and weight section 49 is supported in these beam sections 47 and 48. The movable electrode section 50 is formed in one portion of this weight section 49. On the one hand, a pair of fixed electrodes 51 corresponding to one movable electrode section 50 is disposed so that they are facing one another on silicon substrate 41. Then, in the case where an acceleration changes in a parallel direction (shown by Y in FIG. 18) on the surface of silicon substrate 41, the electrical capacitance between movable electrode section 50 and fixed electrode 51 is changed such that the capacitance of one side increases while the other side is reduced. Further, underlying electrode 52 having an impurity diffusion layer is formed in a region in silicon substrate 41 facing movable section 42; by making the electrical potential of this underlying electrode 52 is made equal to the electrical potential of movable section 42, movable section 42 being attracted towards silicon substrate 41 due to electrostatic force produced between silicon substrate 41 and movable section 42 is prevented.
Furthermore, the above differential capacitance semiconductor acceleration sensor is considered to be an improved MIS transistor-type semiconductor acceleration sensor.
The MIS transistor-type semiconductor acceleration sensor according to the prior work of the present inventors will be described with reference to FIGS. 20, 21, 22, and 23. FIG. 20 shows a plan view of the sensor, in FIG. 21 the cross sectional view taken along line F--F of FIG. 20 is shown, in FIG. 22 the cross sectional view taken along line G--G of FIG. 20 is shown, and in FIG. 23 the cross sectional view taken along line H--H of FIG. 20 is shown.
In a description of this sensor, concerning a device which achieves the same functions as the previously mentioned differential capacitance semiconductor acceleration sensor, by affixing the same numbers, that description is omitted. As shown in FIG. 20, movable electrode sections 53 and 54 functioning as gate electrodes are formed in movable section 42. In the meantime, as shown in FIGS. 20 and 22, two pairs of fixed electrodes (source and drain electrodes) 55, 56, 57 and 58, each composed of an impurity diffusion layer, are formed in both sides of movable electrode sections 53 and 54, respectively. Further, as shown in FIGS. 20 and 21, peripheral circuit 59 is formed in silicon substrate 41. This peripheral circuit 59 and beam structure 63 are electrically connected, and peripheral circuit 59 and fixed electrodes 55 to 58 are electrically connected; moreover, peripheral circuit 59 and underlying electrode 52 are electrically connected. More concretely, the electrical connection of peripheral circuit 59 and beam structure 63 is as shown in FIG. 24: a wiring material 60 such as Al--Si is extended from peripheral circuit 59, and this wiring material 60 and beam structure 63 are connected via impurity diffusion region 61. A voltage generated by this peripheral circuit 59 is applied to beam structure 63.
Then, a voltage is applied between beam structure 63 and silicon substrate 41, and a voltage is applied between fixed electrodes 55 and 56, and between fixed electrodes 57 and 58. In this condition, an acceleration changes in a parallel direction (shown by Z in FIG. 20) on the surface of silicon substrate 41, and variation in a current (drain current) between fixed electrodes 55 and 56, and between fixed electrodes 57 and 58, occurs due to displacement of movable electrode sections 53 and 54. This variation in current is measured by peripheral circuit 59 and applied acceleration is detected.
Here, as shown in FIG. 24, the electrical connection between peripheral circuit 59 and beam structure 63 is formed by impurity diffusion region 61. The problem became clear that, by means of this, leakage current occurs from impurity diffusion region 61 to the silicon substrate 41 side, giving rise to a loss of voltage applied to beam structure 63. That is, despite the prescribed voltage being produced by peripheral circuit 59, only the voltage not lost by the leakage current is applied to beam structure 63. Further, as shown in FIG. 23, channel 62 is formed between fixed electrodes (source and drain electrodes) 55 to 58 and underlying electrode 52, wherein leakage current occurs and there is also the problem of deterioration of sensor characteristics.